Define X-Wing constants as expressions.

See https://boringssl-review.googlesource.com/c/boringssl/+/82327/comment/5192b1ef_5bd31e64/
for more info.

Change-Id: I1223c2b0f57ebc65b9fc5060a852a2f505a3c700
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/82367
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>

crypto/hpke/hpke.cc

diff --git a/crypto/hpke/hpke.cc b/crypto/hpke/hpke.cc
index ecaab3b..0a905af 100644
--- a/crypto/hpke/hpke.cc
+++ b/crypto/hpke/hpke.cc
@@ -604,11 +604,11 @@
   return &kKEM;
 }
 
-#define XWING_PRIVATE_KEY_LEN 32
-#define XWING_PUBLIC_KEY_LEN 1216
-#define XWING_PUBLIC_VALUE_LEN 1120
+#define XWING_PRIVATE_KEY_LEN XWING_PRIVATE_KEY_BYTES
+#define XWING_PUBLIC_KEY_LEN XWING_PUBLIC_KEY_BYTES
+#define XWING_PUBLIC_VALUE_LEN XWING_CIPHERTEXT_BYTES
 #define XWING_SEED_LEN 64
-#define XWING_SHARED_KEY_LEN 32
+#define XWING_SHARED_KEY_LEN XWING_SHARED_SECRET_BYTES
 
 static int xwing_init_key(EVP_HPKE_KEY *key, const uint8_t *priv_key,
                           size_t priv_key_len) {